A method for forming an optical grating within a waveguide integrated on a
substrate includes the step of depositing on a substrate successive layers
of material constituting a waveguide such that the waveguide has a
periodically varying width along a portion of its longitudinal axis. The
deposition may be accomplished by depositing by selective area epitaxy at
least some of the successive layers through a mask having a periodically
varying width along at least one edge. The successive layers deposited
through the mask may constitute a plurality of quantum well layers
separated from each other by barrier layers which collectively form a
multiple quantum well stack.

1. A method for forming an optical grating within a waveguide integrated on
a substrate, said method comprising the steps of:

depositing on a substrate successive layers of materials constituting a
waveguide such that a portion of said waveguide has a periodically varying
thickness and width in a direction of propagation defined by its
longitudinal axis, wherein said waveguide portion has an effective
refractive index profile that periodically varies in both directions
transverse to said direction of propagation.

2. The method of claim 1 wherein the depositing step further comprises the
step of depositing by selective area epitaxy at least some of said
successive layers through a mask having a periodically varying width along
at least one edge.

3. The method of claim 2 wherein said mask comprises two strips of
dielectric material.

4. The method of claim 2 wherein said mask comprises a plurality of strips
of dielectric material.

5. The method of claim 2 wherein certain of said successive layers
deposited through said mask comprise a plurality of multiple quantum well
layers separated from each other by barrier layers which collectively form
a multiple quantum well stack.

6. The method of claim 5 further comprising the step of depositing on said
substrate layers of material that form an active device, said active
device having multiple quantum well layers located in a common plane with
said multiple quantum well stack of said waveguide.

7. The method of claim 6 wherein said active device is a laser.

8. The method of claim 6 wherein said active device is of a type selected
from the group consisting of a modulator, switch, tunable filter,
wavelength converter, and an optical detector.

9. The method of claim 5 wherein said multiple quantum well stack is formed
from a material system selected from the group consisting of InGaAs/InP,
InGaAsP,/InP, and InGaAs/InGaAsP.

10. The method of claim 2 wherein said waveguide is a buried
heterostructure waveguide.

11. The method of claim 2 wherein said waveguide is of a type selected from
the group consisting of a ridge-guided waveguide and a strip-loaded
waveguide.

12. The method of claim 1 wherein said thickness and width are periodically
varied according to a period that is substantially constant along the
direction of propagation.

13. The method of claim 1 wherein said thickness and width are periodically
varied according to a period that varies along the direction of
propagation.

14. An integrated optical device comprising:

a substrate; and

a waveguide formed on said substrate, at least a portion of said waveguide
having multiple layers and a periodically varying thickness and width in a
direction of propagation defined by its longitudinal axis such that said
waveguide portion has an effective refractive index profile that
periodically varies in both directions transverse to said direction of
propagation.

15. The device of claim 14 wherein said thickness and width are
periodically varied according to a period that varies along the direction
of propagation.

16. The device of claim 14 wherein said thickness and width are
periodically varied according to a period that is constant along the
direction of propagation.

17. The device of claim 16 wherein said width varies in a serrated manner.

18. The device of claim 14 wherein said multiple layers are a buried
heterostructure.

19. The device of claim 14 wherein said waveguide is of a type selected
from the group consisting of a ridge-guided waveguide and a strip-loaded
waveguide.

20. The device of claim 14 further comprising an active device formed on
said substrate, said active device being located in a common plane with
said waveguide.

21. The device of claim 20 wherein said active device is of a type selected
from the group consisting of a laser, modulator, switch, tunable filter,
wavelength converter, and an optical detector.

22. The device of claim 21 wherein said waveguide and active device each
have a multiple quantum well stack of material located in a common plane,
said multiple quantum well material being selected from the material
systems consisting of InGaAsflnP, InGaAsP/InP, and InGaAs/InGaAsP.

23. The device of claim 14 wherein said multiple layers is a core formed
from a multiple quantum well stack.

24. The device of claim 23 wherein said multiple quantum well stack is
formed from a material system selected from the group consisting of
lnGaAs/lnP, lnGaAsP/lnP, and lnGaAs/lnGaAsP.

25. A method for forming an optical grating within a waveguide integrated
on a semiconductor substrate, comprising the steps of:

(a) depositing a mask comprising at least two strips spaced to form a gap
having a portion with a periodically varying width;

(c) removing the mask so that deposited multiple quantum well material has
a width that varies in the same manner as the gap; and

(d) depositing a material having a refractive index less than the
refractive index of the multiple quantum well material to serve as a
waveguide cladding.

26. A method for forming an optical grating within a waveguide integrated
on a substrate, said method comprising the steps of:

depositing on a substrate successive layers of materials constituting a
waveguide such that said waveguide has a core having a periodically
varying thickness, width, and composition along a portion of its
longitudinal axis.

27. The method of claim 26 wherein said waveguide has a refractive index
profile along said longitudinal axis that periodically varies in both
directions transverse to said longitudinal axis.

28. The method of claim 26, wherein said depositing step includes growing
at least some of said successive layers by selective area epitaxy through
a mask having a periodically varying width along at least one edge.

29. The method of claim 28, wherein said depositing step includes growing
through the mask a plurality of multiple quantum well layers separated
from each other by barrier layers to collectively form a multiple quantum
well stack.

30. The method of claim 29, further comprising a step of depositing on the
substrate layers of material that form an active device, said active
device having multiple quantum well layers located in a common plane with
said multiple quantum well stack of said waveguide.

31. An integrated optical device comprising:

a substrate; and

a waveguide formed on said substrate, said waveguide having a periodically
varying thickness, width, and composition along a portion of its
longitudinal axis.

32. The device of claim 31 wherein said waveguide has a refractive index
profile along said longitudinal axis that periodically varies in both
directions transverse to said longitudinal axis.

33. The device of claim 31, wherein said periodically varying thickness and
width has a period that is constant along said portion of its longitudinal
axis.

34. The device of claim 31, further comprising an active device formed on
said substrate, said active device being located in a common plane with
said waveguide.

35. The device of claim 34, wherein said active device is of a type
selected from the group consisting of a laser, modulator, switch, tunable
filter, wavelength converter, and an optical detector.

36. A method for forming an optical grating within a waveguide integrated
on a substrate, said method comprising the steps of:

depositing on a substrate, by selective area epitaxy through a mask having
a periodically varying thickness and width along at least one edge,
successive layers of materials constituting a waveguide having a
periodically varying thickness and width in a direction of propagation,

wherein certain of said successive layers deposited during said depositing
step form a multiple quantum well stack.

37. The method of claim 36, further comprising a step of depositing on said
substrate layers of material that form an active device, said active
device having multiple quantum well layers located in a common plane with
said multiple quantum well stack of said waveguide.

38. An integrated optical device comprising:

a substrate; and

a buried heterostructure waveguide formed on said substrate, said buried
heterostructure waveguide having a periodically varying thickness and
width along a portion of its longitudinal axis.

39. An integrated optical device comprising:

a substrate;

a waveguide formed on said substrate, said waveguide having multiple layers
and a periodically varying thickness and width along a portion of its
longitudinal axis; and

an active device selected from the group consisting of a laser, modulator,
switch, tunable filter, wavelength converter, and an optical detector.

Description

FIELD OF THE INVENTION

This invention relates to an epitaxial growth method for forming
waveguides, and more particularly to an epitaxial growth method employing
selective area epitaxy to form a waveguide that incorporates an optical
grating.

BACKGROUND OF THE INVENTION

Photonic integrated circuits are typically comprised of a plurality of
photonic devices, located on a semiconductor substrate, that are in
optical communication with one another. Most methods for creating photonic
integrated circuits involve forming one photonic device at a time. This is
due to an inability to regionally vary the bandgap of the quantum well
(QW) material being deposited in a given epitaxial growth.

In the methods noted above, the epitaxial layers required to form a first
type of photonic device, such as a laser, are grown over the whole
substrate. The growth times and source material concentrations used for
the growth are selected so that the quantum well (QW) material that is
deposited has the requisite characteristics, i.e., band gap, to function
as the desired device. The layers are then masked at the region where the
first photonic device is desired. Subsequently, the layers in unprotected
regions are etched away where other devices, such as modulators or
waveguides, are desired. After etching, layers corresponding to a second
type of photonic device are grown on the substrate in the etched regions.
Growth conditions are adjusted for the second growth so that the QW
material exhibits the appropriate band gap. If a third type of photonic
device is desired, the layers are again masked and etched, conditions are
adjusted and a third series of epitaxial layers are grown in the etched
region.

Methods that utilize successive growths as described above may be
collectively referred to as "etch and regrow" methods. Etch and regrow
methods prevent devices such as lasers or other active elements from being
fabricated at the same time and in the same optical plane as other devices
such as waveguides and optical gratings because such devices require QWs
with different bandgaps. Moreover, devices grown from the etch and regrow
method frequently exhibit poor optical interface quality between different
devices, which can result in internal reflections and coupling losses.

Selective area epitaxy (SAE) is an epitaxial growth method that minimizes
the poor optical interface problems associated with the etch and regrow
method. Using SAE, the bandgap of QW material can be varied in the same
plane with a single growth. Thus, layers defining various photonic devices
can be grown simultaneously. See Joyner et al., "Extremely Large Band Gap
Shifts for MQW Structures by Selective Epitaxy on SiO.sub.2 Masked
Substrates," IEEE Phot. Tech. Lett., Vol. 4, No. 9 (Sept. 1992) at 1006-09
and Caneau et al., "Selective Organometallic Vapor Phase Epitaxy of Ga and
In Compounds: A Comparison of TMIn and TEGa versus TMIn and TMGa," J.
Crystal Growth, Vol. 132 (1993) at 364-70, which are both hereby
incorporated by reference.

In the SAE method, dielectric masks, such as SiO.sub.x or SiN.sub.x, are
deposited on a substrate. Such masks typically comprise two strips spaced
to form a gap. Source material for forming the epitaxial layers, such as
indium (In), gallium (Ga), arsenic (As), and phosphorus (P), is typically
delivered via a technique such as metalorganic vapor phase epitaxy
(MOVPE).

Source material arriving from the vapor phase will grow epitaxially in
regions where the mask is open, i.e, the substrate is uncovered. Source
material landing on the mask itself will not readily nucleate. Given the
proper temperature and mask width, most of the source material that lands
on the mask reenters the vapor phase and diffuses, due to the local
concentration gradient, to find an unmasked region.

Compared to a completely unmasked substrate, the QW growth that occurs in
the gap for both InGaAs and InGaAsP epilayers will be thicker, and richer
in indium. This effect is due to the relative diffusion coefficients of In
and Ga under typical MOVPE growth conditions. As the QW layers thicken,
changes occur in the quantum confined Stark effect resulting in longer
wavelength (lower energy band gap) QW material. Increasing indium content
also results in longer wavelength QW material. Thus, from both the
quantum-size effect and change in alloy composition, the QWs in the gap
are shifted to lower energy band gaps than regions far from the mask.
Accordingly, the refractive index of the QWs in the gap are increased
relative to the regions outside the gap. By varying the ratio of the mask
width to the gap width, the composition, and hence the bandgap and
refractive index, of QW material can be varied. In U.S. Pat. No.
5,418,183, for example, a SAE method is employed to fabricate lasers and
passive waveguides so that their respective QWs are located in the same
plane of material.

However, when optical grating, waveguides and other active devices are
fabricated together by etch and regrow techniques, they are either formed
in different planes or with significant loss due to interfacial
reflections in the plane of propagation.

SUMMARY OF THE INVENTION

The present invention provides a method for forming an optical grating
internal to an optical waveguide. That is, the optical grating and
waveguide are formed in the same layer of material and hence in the same
plane. An active device such as a laser also may be fabricated at the same
time and in the same layer.

In accordance with the present invention, a method for forming an optical
grating within a waveguide integrated on a substrate includes the step of
depositing on a substrate successive layers of material constituting a
waveguide such that the waveguide has a periodically varying width along a
portion of its longitudinal axis. The deposition may be accomplished by
depositing by selective area epitaxy at least some of the successive
layers through a mask having a periodically varying width along at least
one edge. The successive layers deposited through the mask may constitute
a plurality of quantum well layers separated from each other by barrier
layers which collectively form a multiple quantum well stack.

The present invention also provides an integrated optical device that
includes a substrate and a waveguide formed on the substrate. The
waveguide has a periodically varying width and thickness along a portion
of its longitudinal axis. The period may vary in an arbitrarily prescribed
manner along the longitudinal axis. Alternatively, the period may be
constant along the longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understood
from the following detailed description of specific embodiments thereof
when read in conjunction with the accompanying figures in which:

FIG. 1 is a simplified schematic diagram of a buried heterostructure
waveguide constructed in accordance with the present invention.

FIGS. 2-5 show top views of exemplary masks through which material may be
deposited to fabricate the waveguide in accordance with the present
invention.

DETAILED DESCRIPTION

While the present invention will be described in terms of an optical
grating that is fabricated internal to a buried heterostructure waveguide
such as shown in FIG. 1, one of ordinary skill in art will recognize that
the present invention is equally applicable to the fabrication of gratings
internal to other waveguide structures such as a strip-loaded waveguide,
for example. A waveguide incorporating an optical grating may be used in a
variety of devices and may serve, for example, as the grating in
grating-coupled waveguides that are used in broadband filters.

In FIG. 1, the exemplary buried heterostructure waveguide 2 includes a
substrate 21 on which a buffer layer 24 is fabricated. A multiple quantum
well (MQW) stack 26 serving as the waveguide core is formed on the layer
24. The MQW stack 26 is buried in a cladding layer 51. As described in
more detail below, an optical grating 27 is formed within the MQW stack
26. An active device 4 also may be fabricated on the substrate 21. The
active device 4 may include any type of optically active structure,
including, for example, lasers, modulators, switches, tunable filters,
wavelength converters, and optical detectors. The waveguide 2 and active
device 4 are fabricated in the same layer by SAE. A technique for
fabricating a waveguide and active device in the same layer by SAE is
shown in U.S. Pat. No. 5,418,183, which is hereby incorporated by
reference, and hence will not be discussed further. The waveguide and
active device may be covered with suitable layers that depend on the
particular nature of the active device.

In accordance with the present invention, the buried heterostructure
waveguide 2 is fabricated by the previously mentioned SAE method in which
a mask is deposited on the layer 24 where the MQW stack 26 will be formed.
By selecting an appropriately configured mask, the MQW stack 26 may be
fabricated so that an optical grating 27 is contained therein.

FIG. 2 shows a top view of an exemplary mask 40 located over the substrate
21 and layer 24. In accordance with the present invention, the mask
defines a periodic gap 41 through which the substrate is exposed. The mask
is preferably formed of dielectric material, including, without
limitation, SiO.sub.x and SiN.sub.x, and in particular, SIO.sub.2. The
mask 40 can be made by any suitable method such as, without limitation,
plasma-assisted chemical vapor deposition, electron beam vaporization or
sputtering.

The material that will be patterned into the mask 40 is typically deposited
to a thickness of about 3000 angstroms and then etched to create the
desired mask configuration. The thickness of the mask 40 is advantageously
about equal to the wavelength of the light used to expose the photoresist
during photolithographic patterning. This results in an improvement in
mask features, i.e., sharp edges, relative to other thicknesses.

The mask 40 comprises a pair of strips whose widths undergo periodic
variations. In FIG. 2 these periodic variations are illustratively shown
as a serration. The variations in the width of gap 41 mirrors the
variation in width of the strips. The width of the two strips forming the
mask 40 and the gap 41 must be sized so that the MQW material grown in the
gap is suitable for forming a waveguide. For example, a low loss
waveguiding layer having an effective bandgap of 1.3 microns (and an
effective refractive index of 3.25) may be formed from a MQW stack 26
composed of alternating layers of InGaAsP barrier material 90 .ANG. thick
having a 1.3 micron band gap and InGaAsP quantum well material 50 .ANG.
thick having a bandgap of 1.6 microns. However, in those regions where the
gap 41 is relatively narrow the effective bandgap may be about 1.4 microns
or greater with an effective refractive index of 3.3 or higher.

While the particular mask shown in FIG. 2 is formed from two serrated
strips, other mask geometries are contemplated by the present invention.
Specifically, any mask geometry may be used which defines a gap that
varies in a periodic manner along the optical propagation direction.
Moreover, the periodicity may be constant or it may be variable. For
example, FIGS. 3-5 show some other mask geometries that are contemplated
by the present invention. In FIGS. 2-5 like reference numerals are used to
identify like elements. FIGS. 4 and 5 show examples of mask geometries
which vary in a nonconstant periodic fashion. The two strips constituting
the mask do not need to be symmetric with respect to one another. For
example, the mask in FIG. 2 may be modified so that only one of the two
strips has a serrated edge while the other strip has a straight edge.
Additionally, as FIGS. 3-5 illustrate, the mask may be configured as a
series of discrete elements rather than two continuous strips. One of
ordinary skill in the art will recognize that the mask geometries in FIGS.
2-5 are shown for illustrative purposes only and that the invention should
in no way be construed as being limited to those particular examples.

After depositing the masks, the MQW stack 26 is grown using the SAE method.
The MQW stack 26 comprises a plurality of QW layers. Each QW layer of the
stack is separated by a barrier layer. Exemplary material systems that may
be used for the MQW stack 26 include InGaAs/InP, InGaAsP/InP, and
InGaAs/InGaAsP. As will be appreciated by those skilled in the art, many
parameters will influence the characteristics of the QW material. It is
well known in the art how to vary such parameters to grow QW material
adapted for a particular application. For an optical waveguide the MQW
stack 26 should be tailored to maximize optical confinement and provide
low loss. Regarding the low loss, free carrier absorption characteristics,
scattering and other loss mechanisms should be considered.

Because of the characteristics of the SAE method previously discussed, the
individual QWs are thicker and have an increased indium content in those
portions of the gap that are relatively narrow relative to those portions
of the gap that are relatively wide. That is, for the particular mask
shown in FIG. 2, the thickness and indium content of the QWs varies in a
periodic manner along the entire gap 41. This variation in thickness and
indium content results in a periodic variation in bandgap energy, and
hence, refractive index. In the fully fabricated waveguide this periodic
variation in refractive index will give rise to an optical grating.

After the MQW stack 26 has been fabricated, the mask 40 is etched away
using a suitable etchant such as HF or gas phase chemical etching. The
stack 26 must then be buried in a relatively low refractive index material
such as InP, which serves as the cladding layer 51 of the buried
heterostructure waveguide shown in FIG. 1. A current blocking layer of
Fe:lnP may also be used for electrical isolation when a laser is
fabricated on the substrate. Details concerning these fabrication steps
subsequent to the formation of the MQW stack 26 are well known and hence
will not be discussed further. Such fabrication details may be found, for
example, in "A Multifrequency WG laser by SAE," IEEE Phot. Tech. Lett.,
Vol. 6, pp.1277-1279, 1994.

An optical wave propagating through the waveguide core defined by the MQW
stack 26 experiences a periodic variation in refractive index caused by
the periodic variation in thickness and width of the waveguide core.
Consequently, the buried heterostructure waveguide fabricated in
accordance with the inventive method effectively incorporates an optical
grating therein. Moreover, the waveguide, grating, and active device may
now all be formed in a single plane. That is, the MQW layers of the
various devices through which the optical wave propagates may all be
located in a common plane. The resulting device exhibits substantially
improved optical interface quality between the various components over the
quality that is obtainable with known etch and regrow methods.

The optical grating fabricated in accordance with the present invention is
a full two dimensional grating since light propagating through the
waveguide in the z direction experiences a refractive index variation in
both the x and y directions caused by the variations in thickness and
width. That is, the waveguide has a refractive index profile along its
longitudinal axis that periodically varies in both directions transverse
to the longitudinal axis. In contrast, when an optical grating is
fabricated in accordance with conventional etch and regrow techniques the
refractive index varies in only one of the two transverse directions,
which may be undesirable due to the resulting birefringence.

While a method for forming a buried heterostructure waveguide that
incorporates an optical grating is described above, it should be
understood that other waveguide configurations may be formed according to
the present invention. For example, other waveguide structures that may
incorporate a grating in accordance with the present invention include
various strip waveguides such as raised strip, ridge-guided, and
strip-loaded waveguides. Such structures are described, for example, in
Integrated Optics, T. Tamir, ed., (Springer-Verlag 1979), pp. 62-63.